This invention relates to metal-modified phenol-aldehyde novolac resins, and more particularly to an improved method for producing them. Such metal-modified novolacs are useful as developing agents for basic colorless chromogenic dye precursors such as Crystal Violet lactone. Color producing systems of this type are useful in manifolding systems (so called pressure sensitive copy paper systems or chemical carbonless copy sheet units) wherein a color-developer, a color precursor and a common liquid solvent for each are maintained in isolation on a surface of a sheeting material carrier such as paper, or in isolation on separate surfaces of a carrier. The color developer may advantageously be applied as a coating to the record sheet, either alone or admixed with mineral particles. Conveniently, the color precursor and the solvent may be encapsulated in microcapsules of the type described in U.S. Pat. No. 2,800,457 (the disclosure of which patent is incorporated herein by reference), and said microcapsules together with a binder applied as a coating to a sheet, i.e. the transfer or coated back (CB) sheet. In this instance, the color developer is coated on a separate sheet usually of paper, i.e. the color receiving or coated front (CF) sheet. When the two sheets are superimposed with their respective coatings in contact and subjected to localized pressure as by a typewriter key, the walls of the microcapsules in the area subjected to pressure are ruptured, and a solution of the color precursor and the solvent originally contained in the ruptured capsules is transferred in a marking pattern to the sheet surface carrying the color developer. Almost instantaneously a colored mark is formed on the surface carrying the color developer. Alternately, the microcapsules and color developer may be intermixed on the same surface of a substrate sheet. If desired, the microcapsules may contain a solvent solution of the color developer, in which event the color precursor is either applied as a coating to the second substrate sheet or is intermixed with the microcapsules on the same substrate surface.
The use of compounds of the novolac resin and a metal as a means of improving the color forming reactions of the novolac resin with the basic colorless, chromogenic dye precursors is known.
According to H. J. Mueller, U.S. Pat. No. 3,737,410, issued June 5, 1973 (assigned to National Cash Register Company) it is known to prepare zinc modified novolac resins by heating in the melt, zinc dibenzoate and the novolac resin in the presence of either a strong base or a weak base such as zinc hydroxide, ammonium carbonate and ammonium bicarbonate.
Also according to F. D. Weaver, U.S. Pat. No. 4,025,490, issued May 24, 1977 (assigned to The Mead Corporation) a novolac resin modified with zinc, copper, cadmium, aluminum, indium, tin, chromium, cobalt or nickel is prepared by melting the novolac resin together with a metal salt of an alkane carboxylic acid in the presence of gaseous ammonia, ammonium hydroxide, or ammonium compound such as ammonium carbonate or bicarbonate to surpress formation of metal oxide. According to the Weaver patent any metal oxide formed in the process prevents that portion of the metal from entering into the modification of the novolac resin and is wasted. It is indicated by Col. 2, lines 44-46 of the Weaver patent that ammonium moieties are incorporated in the metal modified resin.
It has been found according to the present invention that the metal-modified novolac resin compounds prepared in the presence of an ammonia base according to the aforementioned U.S. patents of Weaver and Mueller are complex coordination compounds. Although the exact structure of these compounds is unknown, the compounds contain carboxylate moieties (e.g. alkanoate or benzoate residues) ammonia (i.e. NH.sub.3) or ammonium (NH.sub.4.sup.+) moieties as well as the metal in complex coordination with the novolac resin.
The preparatory procedures disclosed by the aforementioned Mueller and Weaver patents require charging as reactants metal salts of carboxylic acids (i.e. a metal salt of an alkanoic acid is required as reactant by Weaver while a metal salt of an aromatic monocarboxylic acid is required by Mueller). It is well known that such metal carboxylic acid salts are costly to prepare, isolate and purify. Accordingly, a process for preparing the present compounds which avoids charging as a reactant such costly carboxylic acid salts would be highly desirable to minimize the cost of production of these metal-ammonia-carboxylate-novolac compounds.
It is the object of this invention to devise an improved process for preparing metal-modified novolac phenolic resins suitable for use as color developer coatings in pressure sensitive copy paper systems which does not require charging as reactant a carboxylate salt of the metal.